Transcript EIROFORUM_School_2009

Overview Measuring Techniques for
Magnetic Confinement Nuclear Fusion.
by Andrea Murari on behalf of EFDA JET
Many thanks to the Associations because JET
diagnostics are a real collective effort in Europe.
Bilateral agreements with the RF and the US
Many thanks to F.Romanelli,
and M.Watkins
Why
fusion?
Most
exoenergetic
reaction in the
known universe
Highest power
density per Kg
Lowest
emission of
greenhouse
gases
Technically safe
The fusion process
Light nuclei fuse into heavier nuclei
Fusion products; 14.1 MeV neutron and 3.5 MeV alpha particle
For the nuclei to get
close enough to fuse
they have to overcome
the Coulomb barrier.
To achieve this an
alternative are hot
plasmas: a plasma is a
ionised gas (ions and
electrons are separated)
For fusion to be an
energy source, a hot and
dense fuel plasma must
be confined in a tight
volume for long times...
”Magnetic bottle”
Parameters of Fusion Plasmas
Magnetic fusion
plasmas have
temperature
and pressure
higher than
the solar
corona and
temperature
one order of
magnitude
higher than
the Sun core
Region of
Quantum
plasmas
The Joint European Torus (JET): largest Tokamak
JET main parameters
Major radius
3.1 m
Vacuum vessel
3.96m x 2.4m
Plasma volume up to about 100 m3
Plasma current
up to 5 MA
Toroidal field
up to 4 Tesla
Pulses of tens of seconds
JET has some unique
technical and scientific
capabilities:
 Tritium Operation
 Beryllium Handling
 Plasma Volume and
Magnetic Field to confine the
alphas
Tokamak Plasma Overview
Tokamak
plasma topology
The plasma current is sustained by
the change of magnetic flux: the
plasma is somehow the secondary
winding of a transformer
Tokamak heating schemes
IR and visible views of JET
•Data of a visible Fast Camera:
(210 kfps already
demonstrated target 250 kfps)
Visible no plasma
• 1 MJ ELM or an energy loss
time of the order of 200 ms
(Te 1/2) is at least typical of a
hand grenade
IR during plasma
Fusion Plasmas as Open Systems
A Tokamak plasma is an open system, kept out of equilibrium,
fuelled by injection of both energy and mass and therefore
presents all the problems of control of a typical open
system.
Input of energy and matter: fuelling
and additional heating systems
Internal Transformations:
optimization of the plasma
configuration to maintain the
internal structure and maximise
energy production.
Elimination of the waste: power and
particle exhaust.
Contamination: Helium Ash
Goals of Diagnostics
• Obtain the magnetic topology (magnetic and
electric fields)
• Determine the Plasma Energetic Content
(Temperature and Density)
• Measure the Plasma losses (radiation,
particles)
• Determine the flow and turbulence
Final goal
Measure the fusion products, neutrons and
alpha particles, to control the energy
production
JET diagnostics
• All major measurement
techniques in physics are
represented
• At JET about 100 diagnostics
operational and about 20 more in
the design phase
• The measuring instruments are
different but must be
coordinated in a single
experiments
• Already acquired a maximum of
more than 11 GBytes of data per
shot (equivalent to about a 2 hours
digital movie). Database: more
than 40 Tbytes (comparable to
US Congress library)
• All the information is relevant
and should be interpreted.
JET DATA /Computation
Amount of raw data acquired per shot
CPU use (Gigaflops)
•
•
Red Line: Moore
Law
Blue Curve: JET
Experimental data
acquired per shot
(almost 12 Gbytes)
•
JET cluster: Order of 150
Gigaflops
Plasma Diagnostics
Plasmas are very
delicate
physical
systems
Diagnostics are
mainly passive
and measure
the natural
emission from
the plasma
Active probing
can be done
with laser or
particle beams
Solid probes are
possible only at
the very edge
Fusion neutrons
Outline
g-ray and neutron diagnostics:
based on nuclear physics
Spectroscopy ( IR, visible, UV and
SXR): based on atomic physics
Interferometry in the IR, Thomson
Scattering (visible): based on
Classical Electrodynamics
Magnetic topology with pick-up coils
based on Classical Electrodynamics
Particle systems
In Magnetic Confinement Fusion
measurements are performed along
the whole electromagnetic spectrum
Objectives of the talk
Fusion neutrons
Particle systems
•Describe the basic physical
principles behind the main
measurement methods
•Identify the main plasma
parameters which can be
measured
•Show main results and their
validity by cross validation
comparing measurements
obtained with completely
independent measuring
techniques
•Provide an idea of how the
various diagnostics
(independent experiments) are
implemented in real life.
•Highlight some advanced
developments of the techniques
Measuring the Magnetic Topology
Fusion neutrons
Magnetic fields
Coil systems
V
=-
d f
dt
•The traditional measurements of the magnetic
fields outside the plasma are performed with coils
and are based on induction.
Location of the pick-up coils
Hundreds of coils of various nature are
typically located around the vacuum vessel
of a Fusion device and some inside.
Various methods based on Classical
electrodynamics (vacuum) are used to
derive the plasma boundary from the
external magnetic fields
Poloidal cross section
UC
OPLC
DC
Real time determination of the plasma
boundary
Black: vacuum
vessel
Green plasma
boundary.
Red lower X point
Blue Upper X point
Current Inversion for
internal magnetic topology
Plasma and surrounding structures
modelled with a series of current beams
•No assumption on the equilibrium
•Bayesian Statistics to determine the most likely
distribution of the currents given the measurements
of the pick-up coils
MAP Estimates
The Current Tomography based on a Bayesian Estimator works
well for both the limiter and X-point phase of the plasma
Measuring the properties of the
electron fluid
Visible+ near UV- IR
Laser Thomson Scattering (Te, ne)
•Ruby Nd YAG /double Nd YAG
Infrared interferometry &
polarimetry for ne and B
Interferometry
What is measured is the phase difference between a laser
beam (112 mm) crossing the plasma and a second reference
ne profile
beam:
f = re  ne dl
L
where:
re =
e2
15
=
2.82x10
m
4 c2 0 me
E1 cos( t)
E2 cos( t f)
Detector
The phase shift provides the
average electron density along the
Detectors are
V
beam line because the electrons only
Bolometers InSb
working at liquid He
interact with the wave!
f can be determined by V = E12  E22  E1  E2 cos(f )
2
2
interference:
In Fusion interferometers are of the Mach-Zender type
and use a super-heterodyne approach for the detection
FIR diagnostics
Since the electrons are immersed in a strong magnetic field
they constitute an anisotropic medium optically active (due
to their gyration around the field lines).
POLARIMETER
INTERFEROMETER
Plasma
time
time
Probe
Wire grid
t
Reference
Faraday rotation angle
Phase shift between
reference and probe signals

F=
= C   n( z ) dz
2
z
z2
Plasma
1
Linear polarisation
Phase shift between two
orthogonal polarisation
components
Rotated Elliptical
polarisation:
due to the anisotropy
and optical activity of
the medium
KG4 polarimeter
Faraday Rotation effect
The plane of linearly polarised light passing through a plasma is rotated when
a magnetic field is applied PARALLEL to the direction of propagation.
Faraday Rotation angle
    ne B p|| dz
2
Cotton-Mouton effect
The ellipticity acquired by a linearly polarised light passing through a plasma
is dependent on the magnetic field PERPENDICULAR to the direction of
propagation.
Cotton-Mouton angle
    ne Bt dz
2
3
At JET, for the vertical channels, Bt being largely constant along the line of sight is
reducing the previous equation in
   Bt
3
2
n
dz
e

Interferometric Diagnostic at JET
4 vertical channels 14
4 lateral channels 58
Single Colour Interferometer =195mm
2 Colours Interferometer
=195mm (DCN laser) main laser
=118.8mm (Alcohol laser) compensation
laser
1
Comparison with LIDAR:
profile
JET FIR Interferometer/polarimeter
diagnostics in reality
•Optical path =80 m
•Hundreds of optical
components
•FIR power = 200mW
Incoherent Thomson Scattering
LASER

Thomson Scattering
Scattering from free electrons:
contrary to Compton
scattering radiation of low energy:
no change in
the momentum of the particles
PLASMA
DETECTOR
A laser beam is launched to the plasma. The scattered radiation
from a given area is observed with angle .
The spectrum of the scattered radiation carries the information on
the plasma properties (electron density and temperature).
The particles scattering the light independently are the electrons
This diagnostic was used by scientists from Culham in 1968 to
confirm the high temperatures reached in the first Russian
Tokamak, leading to the development of Tokamak devices all over
the world.
Incoherent Thomson Scattering: LIDAR
Broadening gives the electron temperature Te
Absolute intensity gives the electron density ne
Lasers: Ruby and NeYAG
LASER
T2
T1
T3
x=ct
x
t1
t2
1ns->30cm
DETECTION SYS.
In case of radar measurements
time is translated into position
PLASMA
t3
Incoherent Thomson Scattering
LIDAR at JET: principle
Te
ne

Space resolution:
x2= c2.( tlaser2+ tdet2+ tdaq2)/4
Collection
optics
tlaser=300ps
tdet= 300ps
tdaq= 100ps
x=c.t/2
x= 7 cm
Number of photons reaching the detectors can be ten orders of magnitude
lower than the beam. Detectors GaAs (P) specially developed for fast
response and high QE
Thomson scattering
optical path: about 50
meters
• Power density of
about 10 GW (for only
300 picoseconds)
•Frequency of laser
pulses: 20 Hz
Collective Thomson Scattering
Contrary to high energy physics in Fusion the tendency is to use longer
wavelengths to probe the ion fluid and to investigate collective effects.
Fast ions, being fully stripped, are nearly invisible but their
wakes in the electron fluid give them away.
Fast ions draw a wake in the electron distribution, detectable
by Collective Thomson Scattering (CTS). And at scales larger
than the Debye length ion wakes are the dominant cause of
microscopic fluctuations. Measurement of a collective effect.
Blue: wake in the electron
Light scattered coherently
fluid
from the elctrons give
Ion (swan)
information about the presence
of the fats ions.
D
Contrary to the high energy
physics longer laser
wavelengths are required to
detect this collective effect.
Collective Thomson Scattering
Received scattered radiation
ks
k Resolved fluctuations
Scattering volume
ki
Incident radiation
Theoretical CTS
scattering function
Fast ions can be
studied from
νδ= νs - νi ~ 1 GHz
Forward scattering only
viable alternative to
measure the wake of the
fast ions on the electron
fluid
Measuring the properties of the
Ion Fluid and impurities
Visible+ near UV- IR
Plasma ions, being fully stripped, are nearly invisible but the
impurities in the plasma thermalise with the ion fluid and emit
characteristic radiation which can be analysed
spectroscopically.
Passive spectroscopy: from
IR to SXR. Ti, rotation…
Charge-exchange recombination
spectroscopy:
Visible lines for ion fluid and
impurity studies
Need extensive database on atomic
physics
Ion Diagnostic: Charge eXchange Recombination
Spectroscopy (CXRS)
Principle: derive information about the main plasma ion fluid by
measuring the properties of intrinsic impurities which are
thermalised (have the same temperature and rotation as the main
plasma)
Neutral Particle beam
Charge exchange excitation process between an impurity C6+
and the neutrals of the beam
Charge eXchange Process
Charge exchange excitation process
Information derived by CXRS
Principle of Charge Exchange spectroscopy
The emitted radiation carries the
information about C5+
• temperature
• momentum : The CX reaction does
produces very little momentum change for
the recombined ion and hence does not
disturb the ion velocity distribution.
• the number of C6+
Temperature, velocity and impurity measurements
• Broadening dominated by
Doppler width.
Reference
line
V
• Velocity can be measured
from Doppler shift
Ti
•The density of the impurities
can be determined from the
absolute intensity of the line
Core CXRS diagnostic at JET
• Spatial resolution:
limited by crossing
between beam and
los. Order of few cm
•Time resolution:
limited by the
detector ~10ms.
In terms of detectors, spectroscopy in fusion requires
development mainly of spectrometers.
Measuring the parameters of the
Fusion Products
“Burning Plasma” Diagnostics: fusion products
In a “Burning Plasma” additional quantities have to be
measured
The “fuel mixture” or “isotopic composition”:
the maximum performance is expected at 50/50 D/T
He ash
Tritium
retention:
D
T
thermalised
alphas left
in the
plasma
which
dilute the
main fuel
n 14 MeV
a
The 3.5 MeV as
which are meant to
heat the plasma and
sustain the
discharge
3.5 MeV
unburned tritium
left in the
machine (during
TTE 10% T in the
plasma 90 % in
the wall in case of
puffing)
The 14 MeV
neutrons which are
supposed to transfer the
heat outside the vacuum
chamber
Fusion neutrons
Gamma and X-ray systems
Gamma systems
- Measurement of the fast ions
Particle systems
g-ray Emission
g -ray emission in a Tokamak is produced by
 fusion products: p(3 MeV, 15MeV), T (1 MeV), 3He(0.8
MeV), a (3.5 MeV)
 ICRH-accelerated ions: H, D, T, 3He, 4He
due to nuclear reactions with fuel and main impurities (Be, C)
a-particle diagnosis at JET is based
on the 9Be(a,ng)12C reaction
Fast deuterons detection at JET is
based on the12C(d,pg)13C reaction
9Be(a,ng)12C
9
Be + a
13
reaction
*
C
n 12 *
C
g
12
C
The nuclear reaction
between fast alphas with
Ea > 1.7 MeV and Be
impurity leads to:
 Excitation of high-energy
levels in 13C* nucleus
 De-excitation by emitting
neutrons with population
of low-lying levels in 12C*
 Further de-excitation by
3.1-MeV (D) and 4.44-MeV
(a) gammas to ground
state of 12C nucleus
g-ray Detection: Solid State Scintillator CsI(Tl), NaI(Tl)
Conduction Band
Alkali Halide scintillators
Activator excited states
Band
Scintillation
Gap
Photon
Measurements with high resolution
spectrometers for two discharges
with different neutral beam power
input into the fast particles
Activator ground state
12C(d,pg)13C
Valence Band
An energetic photon creates
electron -hole pairs.
 The electrons and holes migrate



9Be(a,ng)12C
to the activator sites (Tl).
De-excitation of the activator
atoms produces radiation more
efficiently.
The light is then detected with
photomultipliers as in the case of
organic scintillators.
The properties of solid state
scintill. depend on the crystal
structure
3.1
MeV
Higher neutral
beam power
4.44
MeV
Lower neutral
beam power
JET Neutron Cameras
Vertical camera: 9 lines-ofsight
Horizontal camera: 10 linesof-sight
Collimators: Ø10 and 21
mm
Space resolution: 15 cm in
centre
Neutron Detectors:
- 19 Liquid scintillators
NE213 (2.45 and 14 MeV)
+ PSD
- 19 Plastic Bicron 418
scintillators (14 MeV)
Detectors for g-rays:
- 19 CsI(Tl) solid state
For every
ofg-rays
sight there
detectors
forline
the
is a collimator and a
complete set of three
detectors one of each
category. Diagnostic
calibrated absolutely.
Visualization of Fast Particles
He in reversed-shear discharge
4
He in monotonic q(r)-plasma
D in monotonic q(r)-discharge
0.8
0.8
0.6
0.6
0.6
0.4
0.4
0.4
0.2
0
Z, (m)
0.8
Z, (m)
Z, (m)
4
0.2
0
0
1.9MeV
-0.2
2.6
2.8
1.9MeV
-0.2
3
3.2
R, (m)
3.4
3.6
0.2
2.6
2.8
1MeV
1.5MeV
2MeV
-0.2
3
3.2
R, (m)
3.4
3.6
2.6
2.8
3
3.2
3.4
3.6
R, (m)
Results (tomography constrained by the equilibrium) are confirmed by simulations and can provide
essential information on the effects of additional heating and magnetic topology on fast particles
Plant diagnostics (not covered)
• Machine diagnostics (thermocouples, strain
gauges etc)
• Vacuum Diagnostics (gauges etc)
• Cryogenic diagnostics (low Temperatures, etc)
• Tritium reprocessing plant (RGA etc)
• Surface and thin film analysis
Summary
• Magnetic fusion plasmas are complex, open systems, kept out
of equilibrium to maximise performance
• Magnetic Confinement Fusion is one of the fields of physics
which requires the highest number of different diagnostics (all
major measuring techniques of physics are represented).
• This variety is due to the fact that
– the temperature range covered is enormous (from liquid He
to 100 million 0C)
– the density range is also significant (from 1 bar to UHV 10-810-9 mbar)
• The amount of data available is quite remarkable. Since nuclear
plasmas require an holistic understanding (more like the Health
Sciences than the High Energy Physics) the interpretation task
is formidable.
Gamma
Ray
GRS (Gamma
Ray Spectroscopy
Spectroscopy)
Gamma Ray Spectroscopy (GRS) project: gamma spectroscopy at
high rate and high energy resolution
Three new detectors will be installed at JET. Two are based on
inorganic scintillator (featuring very good light yield and fast time
decay), the third one is a HpGe and has been installed on JET.
KM6G is a High-resolution and High efficiency HPGe spectrometer
(about a factor of 20 better energy resolution than NaI).
High efficiency germanium detector
Relative photopeak efficiencies: 100%
High resistance to neutron damage: N-Type detector
Energy range for spectroscopy measurements from 50 keV to 10 MeV
Energy resolution: less than 2.8 keV at 1.33 MeV
Peak to Compton Ratio: above 60:1
Mechanical cooling system
Mechanical Cooling system for the Ge crystals which can
replace the standard Liquid Nitrogen (LN2) dewar.
First measurement of gamma ray Doppler
broadening in fusion plasmas!
C12(He3,p)14N
Wide spectrum shows
very well resolved lines
Co60
120
73769 (c/bin)
2313
100
Co60
1635
Be9(He3,n)11C
Be9(a,n)12C
80
60
K40
2000
C12(He3,p)14N
y = m4+m1*exp(-(m0-m2)*(m0-m...
4440 SE
4440 DE
4440
40
5104
20
1000
1635 keV g line
2000
3000
4000
5000
Value
Error
m1 120
55,948
3,3614
m2
1636,3
0,77799
m3
13,307
0,84483
m4 100
22,954
1,1089
Chisq
44,528
NA
R
0,93923
NA
6000
73769 c/bin
80
Energy (keV)
60
40
Modelling of individual line
broadening
20
0
1580
1600
1620
1640
Energy (keV)
1660
1680
1700
Temperature of the tails
<T3He> = 0.27± 0.04MeV
Pulse #73764
55
50
45
b
r
o
a
d
e
n
i
n
g
Preliminary results
(lines 1635 keV and 2313 keV
60
*
of the reaction C12 (He3, p)14N )
46+/-1.7
keV
Line emission coherent with
Total yield
9Be(a,ng) 12C
40
35
28+/-1 keV
30
25
*
20
15
30
130
230
330
430
Temp of the tail (Gaussian)
First measurements in a
Tokamak of tail temperature
brodeneing with gamma rays:
confirmed by different lines
530
4.439-MeV g-ray yield, a.u.
L
i
n
e
3
3x10
9
12
Be(a,ng) C
3
2x10
3
1x10
0
100
150
3
200
250
D He fusion yield, a.u.
300
Principles of Neutron Detection
Since its discovery in 1932 by Chadwick, the neutron is well
known for being an elusive particle.
The main method to detect neutrons consists of “transforming” them (via
nuclear processes:strong interactions) to charged particles, which
then interact with matter through Coulomb collisions.
Target nucleus +
neutron
• Recoil nucleus
(proton, elastic)
• Proton
Conversion
• Alpha particle
reactions
• Fission fragments
In fusion fast neutrons (E > 100 keV) have to be detected and the main
methods used rely on:
•
Recoil protons
scintillators: the recoil protons excite suitable materials
which in turn emit light collected by a photomultiplier
•
Conversion reactions producing as (n,a)
in semiconductors the reaction products create electronhole pairs and the charge is collected (Si or Diamond
detectors)
•
Induced fission in materials (n,fission) : fission chambers

Neutron Emission Spectrometry
The objective is the measurement of the neutron energy
spectra (not only the number of neutrons)
Neutrons generate recoil protons in
foil (converter) and then the protons
are analysed in momentum by a
magnetic field
Neutrons generate light in a first
organic scintillator (scatterer) and
then in a second: neutron energy
determined by the time of flight)
Neutrons generate recoil protons in
an organic scintillator and their
deposited energy is determined by
measuring the amount of emitted
light (L.Bertalot poster P1.078)
Neutron spectrum of thermal plasma
•Spectral width:
•Broadening  Ti
•Peak position:
•Energy shift
vtor
•Area, Cn PF
•
Yn  WF
•Total absolute
calibration
• Absolute
Magnetic Proton Recoil (MPR) spectrometer
14 MeV neutrons
calibration from
first principles
• Flexible
settings and high
reliability and
stability
• Well known
response function
• High
Energy resolution
Total 14 MeV neutron yield (with profile factor from
neutron cameras)
Velocity of the ions depending on ICRH phasing
Velocity of ions
depends on ICRH
phasing, the
direction the
heating waves are
injected into the
plasma
Dipole, ES = 21±15 keV --> vtor = 57 ± 41
km/s
90°, ES = 114±13 keV --> vtor = +309 ± 36
km/s
-90°, ES = -103±15 keV --> vtor = -279 ± 41
km/s
Upshifted
Downshifted